Palladium-containing composition and hydrogen peroxide production method

ABSTRACT

It is desired to develop a method of producing hydrogen peroxide, which is capable of producing hydrogen peroxide with high production efficiency. According to the present invention, provided is a palladium-containing composition comprising palladium particles and a coating agent that coats the surface of the palladium particles, wherein a compound having an O═X structure (wherein X represents any of a phosphorus atom, a sulfur atom, and a carbon atom) is comprised as the coating agent.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a 35 U.S.C. § 371 national stage applicationof International patent application PCT/JP2019/035846, filed Sep. 12,2019, which is based on and claims the benefit of priority to JapaneseApplication No. 2018-171847, filed on Sep. 13, 2018. The entire contentsof these applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a palladium-containing composition, anda method of producing hydrogen peroxide, in which thepalladium-containing composition is used, and hydrogen and oxygen areallowed to directly react with each other to obtain hydrogen peroxide.

BACKGROUND ART

Since hydrogen peroxide has oxidizing power and strong bleaching and/orbactericidal action, it is used as a bleaching agent and/or abactericide for papers, pulps, fibers, and the like. In addition,hydrogen peroxide is an important industrial product used in a widerange of oxidation reactions including epoxidation and hydroxylation astypical examples.

Furthermore, hydrogen peroxide is used for the cleaning of the surfaceof semiconductor substrates, etc. in the semiconductor industry, thechemical polishing of the surface of copper, tin and other copperalloys, the etching of electronic circuits, and the like. Since thedegradation products of hydrogen peroxide are water and oxygen, hydrogenperoxide holds a prominent position also from the viewpoint of greenchemistry, and thus, hydrogen peroxide has attracted attention,particularly, as a material alternative for chlorine-bleaching agents.

As methods of producing hydrogen peroxide, an anthraquinone process, anelectrolytic process, a method involving oxidation of isopropyl alcohol,etc. have been known. Conventionally, the anthraquinone process has beenmainly adopted in the industrial field. However, since the anthraquinoneprocess is a multi-stage method comprising multiple stages such ashydrogenation of anthraquinone, air oxidation, extraction of thegenerated hydrogen peroxide with water, and further, purification, andconcentration, this process is problematic in terms of high capitalinvestment, the use of a large amount of energy, the release of anorganic solvent used for dissolving anthraquinone into the air, etc.

As a method of solving the above-described problems, Patent Literature 1proposes that a precious metal colloidal solution, in which colloidalparticles containing a precious metal are dispersed, are allowed tocoexist and react in a reaction system in a method of directlysynthesizing hydrogen peroxide in which hydrogen is allowed to reactwith oxygen (which is also referred to as a “direct production method”).Patent Literature 1 describes that hydrogen peroxide can be producedwith high production efficiency according to the aforementioned method.However, there has been plenty of room for improvement in terms ofproduction efficiency.

CITATION LIST Patent Literature

-   -   Patent Literature 1: JP Patent Publication (Kokai) No.        2005-272255 A

SUMMARY OF INVENTION Technical Problem

In view of the foregoing, it has been desired to develop a method ofproducing hydrogen peroxide, which is capable of producing hydrogenperoxide with high production efficiency.

Moreover, it has also been desired to develop a palladium-containingcomposition, which can be used in the above-described production method.

Solution to Problem

Specifically, the present invention is as follows.

-   -   <1>

A palladium-containing composition comprising palladium particles and acoating agent that coats the surface of the palladium particles, wherein

-   -   a compound having an O═X structure (wherein X represents any of        a phosphorus atom, a sulfur atom, and a carbon atom) is        comprised as the coating agent.    -   <2>

The palladium-containing composition according to the above <1>, whereinthe compound further has an aryl group or an alkyl group containing 1 to10 carbon atoms.

-   -   <3>

The palladium-containing composition according to the above <1> or <2>,wherein, in the compound, X represents a phosphorus atom or a sulfuratom.

-   -   <4>

The palladium-containing composition according to any one of the above<1> to <3>, wherein the compound is triphenylphosphine oxide, diphenylsulfoxide, or diphenyl sulfone.

-   -   <5>

The palladium-containing composition according to any one of the above<1> to <4>, wherein the palladium particles are palladium colloidalparticles.

-   -   <6>

The palladium-containing composition according to the above <5>, whereinthe particle diameter of the palladium colloidal particles is 1 to 10nm.

-   -   <7>

The palladium-containing composition according to any one of the above<1> to <6>, which further comprises platinum particles.

-   -   <8>

The palladium-containing composition according to the above <7>, whereinthe platinum particles are platinum colloidal particles, and theplatinum colloidal particles is coated with the coating agent.

-   -   <9>

The palladium-containing composition according to any one of the above<1> to <8>, which is used as a catalyst in the production of hydrogenperoxide by reacting hydrogen with oxygen.

-   -   <10>

A palladium-containing solution comprising the palladium-containingcomposition according to any one of the above <1> to <9> and an organicsolvent.

-   -   <11>

The palladium-containing solution according to the above <10>, whereinthe concentration of palladium is 5 mmol/L or more.

-   -   <12>

A method of producing hydrogen peroxide, comprising reacting hydrogenwith oxygen to obtain hydrogen peroxide, wherein

-   -   the palladium-containing composition according to any one of the        above <1> to <9> is used as a catalyst.    -   <13>

The production method according to the above <12>, which comprises:

-   -   mixing the palladium-containing composition with an organic        solvent to prepare a palladium-containing solution,    -   mixing the palladium-containing solution with water or an        aqueous solution to prepare a mixed solution, and    -   supplying hydrogen and oxygen into the mixed solution to        generate hydrogen peroxide.    -   <14>

The production method according to the above <13>, wherein

-   -   the organic solvent is a benzene derivative, and    -   upon preparation of the mixed solution, the palladium-containing        solution is mixed with a sodium bromide aqueous solution.    -   <15>

The production method according to the above <14>, wherein the organicsolvent is toluene.

-   -   <16>

The production method according to any one of the above <13> to <15>,wherein, in the reaction solution after generation of hydrogen peroxide,the palladium particles are present in an organic layer and the hydrogenperoxide is present in a water layer.

-   -   <17>

The production method according to any one of the above <12> to <16>,wherein only the palladium-containing composition is used as a catalyst.

-   -   <18>

The production method according to any one of the above <13> to <17>,wherein the water or aqueous solution is an acid aqueous solution havinga pH value of 0.5 to 2.0.

Advantageous Effects of Invention

In the palladium-containing composition of the present invention,palladium particles can be converted to be hydrophobic due to the O═Xstructure of a coating agent. Thus, if the palladium-containingcomposition of the present invention is used as a catalyst, for example,in a direct production method of hydrogen peroxide, a mixed solvent ofan organic solvent and water or an aqueous solution can be used as areaction solvent, and as a result, hydrogen peroxide can be producedwith high efficiency.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 includes photographs showing that an example of apalladium-containing solution is mixed with water or an aqueous solutionto obtain a mixed solution, which is then left at rest. FIG. 1(a) showsa mixed solution as a whole. FIG. 1(b) is a TEM photograph showing anorganic layer.

DESCRIPTION OF EMBODIMENTS

In one aspect of the present invention, hydrogen peroxide is producedaccording to a direct production method of reacting hydrogen withoxygen. In the direct production method, a palladium catalyst is used.

In the present invention, as a palladium catalyst, apalladium-containing composition comprising palladium particles and acoating agent that coats the surface of the palladium particles(hereinafter also referred to as “the palladium-containing compositionof the present invention”) is used.

<Palladium-Containing Composition>

The palladium-containing composition of the present invention isobtained by mixing a known palladium particle catalyst with a coatingagent.

Examples of known palladium particles may include particles of palladiumitself; palladium salts, such as palladium acetate, palladium chloride,palladium nitrate, and palladium(II) acetylacetonate; and palladiumcomplex salts, such as ammonium tetrachloropalladate(II) andtetraamminepalladium(II) chloride monohydrate.

The palladium particles are preferably palladium colloidal particles.The specific mean particle diameter is not particularly limited, and ingeneral, it is preferably 1 to 10 nm, more preferably 1 to 7 nm,particularly preferably 1 to 6 nm, and most preferably 3 to 6 nm. Themean particle diameter can be obtained by measuring any given 100particles shown in a TEM photograph in terms of diameter, and thencalculating a mean value of the diameters.

The coating agent plays a role for chemically modifying the surface ofthe palladium particles, and it is also referred to as a protectingagent. In the present invention, it is important to use an agentcomprising a compound having an O═X structure (wherein X represents anyof a phosphorus atom, a sulfur atom, and a carbon atom) as a coatingagent (hereinafter, also abbreviated as a “coating agent having an O═Xstructure”).

Conventionally, according to the direct production method, hydrogen isallowed to react with oxygen in water, in which a palladium catalyst ispresent, so as to generate hydrogen peroxide. According to this method,however, even though various efforts had been made to increase the speedof generating hydrogen peroxide, good results had not been obtained. Asa result of intensive studies conducted by the present inventors, it wasfound that one of the causes would be a problem regarding decompositionof hydrogen peroxide that the generated hydrogen peroxide (H₂O₂) iscontacted with a palladium catalyst, so that it becomes radicals (·OH)and the radicals are then reacted with hydrogen, and thereby they becomewater. In order to avoid the contact of the palladium catalyst with thehydrogen peroxide to the maximum, it is effective that a mixed solutionof water and an organic solvent is used as a solvent, the palladiumcatalyst is unevenly distributed in an organic layer, and the hydrogenperoxide is unevenly distributed in water. For this reason, in thepresent invention, palladium particles are coated with a coating agenthaving an O═X structure, so that the palladium particles are convertedto be hydrophobic (non-aqueous system).

Specific examples of the compound having an O═X structure may includetriphenylphosphine oxide, trioctylphosphine oxide,methyl(diphenyl)phosphine oxide,trans,trans-1,5-diphenyl-1,4-pentadien-3-one, diphenyl sulfoxide, anddiphenyl sulfone.

From the viewpoint of easy availability, it is preferable that thecompound having an O═X structure further has an aryl group or an alkylgroup containing 1 to 10 carbon atoms.

X in the O═X structure is preferably a phosphorus atom or a sulfur atombecause it causes a particularly high speed of generating hydrogenperoxide. Specifically, the compound having an O═X structure ispreferably triphenylphosphine oxide, diphenyl sulfoxide, or diphenylsulfone, and is more preferably diphenyl sulfoxide or diphenyl sulfone.The reason why the speed of generating hydrogen peroxide becomesparticularly high when these compounds are used as coating agents isunknown. The present inventors have assumed that polarizability biaswould influence on an increase in the speed of generating hydrogenperoxide. That is to say, the inventors have considered that a compoundhaving a high electron density and a high polarizability in the O═Xstructure tends to cause a high speed of generating hydrogen peroxide.

The amount of the coating agent having an O═X structure is determined,as appropriate, depending on the molecular weight of the coating agenthaving an O═X structure, etc. The coating agent is used in an amount ofpreferably 50% to 10,000% by mass, more preferably 100% to 7,500% bymass, further preferably 100% to 4,000% by mass, particularly preferably100% to 2,000% by mass, and most preferably 100% to 1,000% by mass, withrespect to the mass of the palladium particles. If the amount of thecoating agent used is too small, it is likely that the effect ofimparting affinity for the organic solvent is decreased. On the otherhand, if the amount of the coating agent used is too large, it does notprovide significant advantages, and also, it provides a disadvantagethat is an increase in the production costs.

Moreover, from the viewpoint of further activating generation ofhydrogen peroxide, it is preferable to mix platinum particles into thepalladium-containing composition of the present invention. Examples ofthe platinum particles may include particles comprising: platinumcomplexes having platinum as a central metal and also having, as aligand, porphyrin, phenylpyridine, bipyridine, terpyridine, salen,phenylpyridine, acetylacetonate, etc.; chloroplatinic acid; andcis-diaminedichloro-platinum(II). The platinum particles are preferablybis(acetylacetonato)platinum (II) particles.

The platinum particles are particularly preferably platinum colloidalparticles. The specific mean particle diameter is not particularlylimited, and in general, it is preferably 1 to 10 nm, more preferably 1to 7 nm, particularly preferably 1 to 6 nm, and most preferably 3 to 6nm. The surface of the platinum colloidal particles is also preferablycoated with the aforementioned coating agent.

Furthermore, a reducing agent for reducing Pd(II) to Pd(0) is preferablymixed into the palladium-containing composition of the presentinvention. Examples of the reducing agent may include oleic acid,hydrazine, NaBH₄, and alcohol. Among these, oleic acid is preferable assuch a reducing agent.

The amount of the reducing agent used is determined, as appropriate. Ingeneral, the reducing agent is preferably used in an amount of 5% to10,000% by mass with respect to the mass of the palladium particles.

The palladium-containing composition of the present invention isproduced by mixing palladium particles with a coating agent and thenstirring the mixture in the presence of an organic solvent at atemperature of generally 20° C. to 100° C., preferably 30° C. to 80° C.,and particularly preferably 40° C. to 60° C. When platinum particles areused, the palladium particles and the platinum particles are mixed withthe coating agent, and the mixture is then stirred.

As an organic solvent used herein, a known organic solvent may beselected, as appropriate, and may be used. For example, chloroform,acetone, acetonitrile, carbonic acid ester, etc. may be used.

The organic solvent may be used in an amount sufficient for dissolvingPd salts therein. The organic solvent is used in an amount of preferably500% to 30,000% by mass, more preferably 1,000% to 20,000% by mass, andparticularly preferably 1,000% to 3,000% by mass, with respect to themass of the palladium particles.

When platinum particles are used, palladium particles:platinum particles(molar ratio) is preferably 99:1 to 50:50, more preferably 99:1 to90:10, and particularly preferably 99:1 to 95:5.

<Production of Hydrogen Peroxide>

The thus obtained palladium-containing composition, preferably, only thepalladium-containing composition is used as a catalyst, and hydrogen isallowed to react with oxygen according to an ordinary method, so thathydrogen peroxide can be produced.

Preferably, a palladium-containing composition is mixed with an organicsolvent to prepare a palladium-containing solution, and the obtainedpalladium-containing solution is then mixed with water or an aqueoussolution to prepare a mixed solution. Thereafter, hydrogen and oxygenare supplied into the mixed solution, so that hydrogen peroxide may begenerated in the solution.

As previously described above, if a reaction solvent prepared by mixingan organic solvent with water or an aqueous solution is used, upon areaction of generating hydrogen peroxide, palladium particles arepresent in an organic layer, whereas the generated hydrogen peroxide isdissolved in a water layer. Thus, the contact of palladium particleswith hydrogen peroxide can be avoided to the maximum by separating thepalladium particles from the hydrogen peroxide during the reaction, anddecomposition of the hydrogen peroxide can be suppressed. In fact, inthe after-mentioned Examples, as a result of suppression ofdecomposition of hydrogen peroxide, it is demonstrated that the speed ofgenerating hydrogen peroxide that indicates the molar amount of hydrogenperoxide generated per unit time is significantly high. In addition, the“selectivity” that indicates the ratio of the amount of hydrogen used ingeneration of hydrogen peroxide to the amount of hydrogen consumed isalso improved. After termination of the generation reaction, a highconcentration of hydrogen peroxide is dissolved in the water layer.

Moreover, when the palladium-containing composition comprises platinumparticles, hydrogen peroxide tends to be more activated. The reason isnot clear, but it is assumed that activation of hydrogen is furtherpromoted due to the presence of platinum particles.

The organic solvent used to prepare a palladium-containing solution canbe either a non-polar solvent or a polar solvent. Examples of thenon-polar solvent may include: aromatic hydrocarbons, specifically,benzene; benzene derivatives comprising an alkyl substituent containing1 to 5 carbon atoms; quinone derivatives; hydroquinone derivatives; andmethylnaphthalene. Examples of the benzene derivative may includetoluene, butylbenzene, pseudocumene (1,2,4-trimethylbenzene),1,3,4-trimethylbenzene, 1,2,5-trimethylbenzene, mesitylene(1,3,5-trimethylbenzene), tert-butylbenzene, and tert-butyltoluene.Examples of the polar solvent may include higher alcohols such asdiisobutylcarbinol, carboxylic acid ester, tetra-substituted urea,cyclic urea, and trioctyl phosphoric acid. For preparation of thepalladium-containing solution, the organic solvent may be used alone asa single type, or may also be used in combination of two or more types.The organic solvent is preferably a benzene derivative, and isparticularly preferably toluene, from the viewpoint that it is anorganic solvent that has previously been used in the anthraquinoneprocess.

The amount of the organic solvent used in preparation of thepalladium-containing solution may be determined, so that the ratiobetween the organic layer and the water layer can be within theafter-mentioned range in the mixed solution.

From the viewpoint of the palladium-containing solution that effectivelyfunctions as a catalyst, the concentration of palladium in thepalladium-containing solution is preferably 5 mmol/L or more, morepreferably 10 mmol/L or more, and particularly preferably 30 mmol/L ormore. The upper limit of the palladium concentration is not particularlylimited, and it is preferably 100 mmol/L or less, more preferably 80mmol/L or less, and particularly preferably 50 mmol/L or less.

When the palladium-containing composition comprises platinum particles,the concentration of platinum in the palladium-containing solution ispreferably 0.2 mmol/L or more, more preferably 0.4 mmol/L or more,further preferably 1.1 mmol/L or more, and particularly preferably 1.2mmol/L or more. The upper limit of the platinum concentration is notparticularly limited, and it is preferably 4.0 mmol/L or less, morepreferably 3.5 mmol/L or less, and particularly preferably 2.0 mmol/L orless.

The water or the aqueous solution used herein is preferably an aqueoussolution containing sodium bromide (NaBr). The concentration of sodiumbromide is preferably higher than 0 mmol/L and 100 mmol/L or less, andparticularly preferably 5 to 50 mmol/L.

When the water or the aqueous solution is a sodium bromide-containingaqueous solution, it is preferable that the organic solvent be a benzenederivative, and that upon preparation of a mixed solution, thepalladium-containing solution be mixed with the sodium bromide aqueoussolution.

Phosphoric acid (H₃PO₄) may be further dissolved in the aqueoussolution. In such a case, the concentration of phosphoric acid ispreferably higher than 0 mmol/L and 10 mmol/L or less. When phosphoricacid is dissolved in the aqueous solution, the aqueous solution isbiased to the acidic side.

The acid aqueous solution is preferable because it tends to suppress thedecomposition rate of hydrogen peroxide and to accelerate generation ofhydrogen peroxide. The pH (25° C.) of the acid aqueous solution is lessthan 7, and is preferably less than 3.5. From the viewpoint of higheraccumulation of hydrogen peroxide, the pH of the acid aqueous solutionis particularly preferably 0.5 to 2.0. When the pH of the acid aqueoussolution is set to be pH 3 or less, and in particular, pH 1 or less,strong acid such as sulfuric acid or hydrochloric acid is preferablyused in addition to the aforementioned phosphoric acid, and inparticular, sulfuric acid is preferably used. For example, in the caseof using sulfuric acid, the concentration of the sulfuric acid ispreferably 0.01 to 10 mol/L.

Photographs regarding an example of the mixed solution before additionof hydrogen and oxygen are shown in FIG. 1 . As understood from FIG.1(a), the mixed solution is a two-phase system of a water phaseconsisting of water or an aqueous solution and an organic phaseconsisting of a palladium-containing solution. FIG. 1(b) is a TEMphotograph of the organic layer shown in FIG. 1(a). In the organic layerof FIG. 1(b), palladium particles are agglutinated to form colloidalparticles, and the circumference of the colloidal particles is coatedwith a coating agent. The palladium colloidal particles coated with thecoating agent are uniformly dispersed in the organic solvent.

With regard to the ratio (volume ratio) between the organic layer (apalladium-containing solution) and the water layer (a water or anaqueous solution) in the mixed solution, the organic layer: the waterlayer is preferably 15:1 to 0.04:1, and is more preferably 3.5:1 to0.35:1. If the volume of the water layer is too large, the concentrationof hydrogen peroxide in the water layer is likely to become low aftertermination of the reaction.

After completion of the reaction of generating hydrogen peroxide bysupplying hydrogen and oxygen, the organic layer is separated from thewater layer, and thereafter, a hydrogen peroxide aqueous solution isobtained from the water layer according to an ordinary method.

In the present description, a method of producing hydrogen peroxide byusing a mixed solution of water and organic solvent is mainly explained.However, even if the palladium-containing composition of the presentinvention is used as a catalyst in the conventional direct productionmethod of using an aqueous solvent, the efficiency of producing hydrogenperoxide that is excellent to a certain extent can be realized.

Moreover, in the present description, the case of using thepalladium-containing composition of the present invention as a catalystfor producing hydrogen peroxide is mainly explained. However, thepalladium-containing composition of the present invention can bepreferably used even for other intended uses, as long as the presentpalladium-containing composition that is characterized in that itcomprises hydrophobic (non-aqueous system) palladium particles can beutilized. For example, the present palladium-containing composition canbe used as a catalyst in other chemical reactions.

EXAMPLES

Hereinafter, the present invention will be more specifically describedin the following examples. However, the present invention is not limitedto these examples.

<Measurement of the Concentration of Hydrogen Peroxide in Water Layer>

The water layers obtained in individual Examples and ComparativeExamples were analyzed using a hydrogen peroxide automatic titrator(manufactured by Hiranuma Sangyo, Co., Ltd., Sterilization CleaningLiquid Concentration Counter Series, Hydrogen Peroxide Counter HP-300)based on an iodine coulometric titration method, and the concentration(number of moles) of hydrogen peroxide in the water layer was obtained.

<Measurement of the Amount of Hydrogen Contained in Discharged Gas fromthe Reaction System>

Discharged gas from the reaction system was analyzed by gaschromatography (GC-TCD), and the amount of hydrogen (number of moles)contained in the discharged gas was obtained.

-   -   Measurement apparatus: Shimadzu Corporation GC-8A    -   Measurement conditions: carrier gas: Ar; column: Molecular Sieve        5A 2 m; temperature: room temperature        <Calculation of the Speed of Generating Hydrogen Peroxide>

The value (number of moles) of the concentration of hydrogen peroxide inthe water layer obtained in each of the examples and the comparativeexamples and the reaction time (h) in each of the examples and thecomparative examples were introduced into the following equation tocalculate the speed of generating hydrogen peroxide (mmol/h):The speed of generating hydrogen peroxide (mmol/h)=the concentration[mmol] of hydrogen peroxide generated in the water layer/the reactiontime [h].<Calculation of Hydrogen Conversion Rate>

In each of the examples and the comparative examples, the value of theamount (number of moles) of hydrogen gas contained in discharged gas andthe value of the amount (number of moles) of hydrogen gas supplied wereintroduced into the following equation to calculate the percentage (%)of hydrogen consumed in the reaction of producing hydrogen peroxide.Specifically, the hydrogen conversion rate indicates the percentage ofhydrogen consumed in the reaction of producing hydrogen peroxide:Hydrogen conversion rate (%)=(1−the amount (number of moles) of hydrogengas in discharged gas/the amount (number of moles) of hydrogen gassupplied)×100.<Calculation of Selectivity>

The speed of consuming hydrogen gas [mmol/h] was calculated according tothe following equation:The speed [mmol/h] of consuming hydrogen gas=(the amount [mmol] ofhydrogen gas supplied−the amount [mmol] of hydrogen gas in dischargedgas)/reaction time [h].

Selectivity (%) in each of the examples and the comparative examples wasobtained according to the following equation:Selectivity [%]=(the speed [mmol/h] of generating hydrogenperoxide)/(the speed [mmol/h] of consuming hydrogen gas)×100.

Specifically, selectivity indicates the percentage of hydrogen used inthe synthesis of hydrogen peroxide, in the hydrogen consumed in thereaction producing the hydrogen peroxide.

Example 1

To a two-necked eggplant shaped flask, 112 mg of palladium acetate (0.5mmol; molecular weight of palladium acetate: 224.51) and 2.78 g oftriphenylphosphine oxide (10 mmol; molecular weight oftriphenylphosphine oxide: 278.29) were added. Further, 4.0 mL ofchloroform was added thereto, and the obtained mixture was then stirredat 50° C. for dissolution. Subsequently, 3.2 mL of oleic acid was addedto the reaction mixture, and the obtained mixture was then stirred at50° C. for 1 hour while heating to obtain a black solution. Thereafter,heating was terminated, and the obtained solution was then cooled toroom temperature. Hence, a palladium-containing composition wasobtained. To the obtained palladium-containing composition, toluene wasadded to result in a total amount of 45 mL, thereby obtaining apalladium-containing solution A comprising triphenylphosphineoxide/palladium nanocolloidal particles. The content of palladium in thepalladium-containing solution A, which was calculated from the amount ofthe palladium acetate added and the amount of the solution (i.e., theaforementioned “total amount”), was 11.1 mmol/L. Thepalladium-containing solution was observed using TEM, and the size ofthe palladium nanocolloid was measured. As a result, the average sizewas measured to be 5.9 nm.

Example 2

A palladium-containing solution B comprising trioctylphosphineoxide/palladium nanocolloidal particles was obtained in the same manneras that of Example 1, with the exception that 3.87 g (10 mmol) oftrioctylphosphine oxide was used instead of 2.78 g of triphenylphosphineoxide. The content of palladium in the palladium-containing solution B,which was calculated from the amount of the palladium acetate added andthe amount of the solution, was 11.1 mmol/L.

Example 3

A palladium-containing solution C comprising methyl(diphenyl)phosphineoxide/palladium nanocolloidal particles was obtained in the same manneras that of Example 1, with the exception that 2.16 g (10 mmol) ofmethyl(diphenyl)phosphine oxide was used instead of 2.78 g oftriphenylphosphine oxide. The content of palladium in thepalladium-containing solution C, which was calculated from the amount ofthe palladium acetate added and the amount of the solution, was 11.1mmol/L.

Example 4

A palladium-containing solution D comprisingtrans,trans-1,5-diphenyl-1,4-pentadien-3-one/palladium nanocolloidalparticles was obtained in the same manner as that of Example 1, with theexception that 2.34 g (10 mmol) oftrans,trans-1,5-diphenyl-1,4-pentadien-3-one was used instead of 2.78 gof triphenylphosphine oxide. The content of palladium in thepalladium-containing solution D, which was calculated from the amount ofthe palladium acetate added and the amount of the solution, was 11.1mmol/L.

Example 5

To a two-necked eggplant shaped flask, 112 mg (0.50 mmol) of palladiumacetate and 2.78 g (10 mmol) of triphenylphosphine oxide were added.Further, 4.0 mL of chloroform was added thereto, and the obtainedmixture was then stirred at 50° C. for dissolution. Subsequently, 3.2 mLof oleic acid was added to the reaction mixture, and the obtainedmixture was then stirred at 50° C. for 1 hour while heating to obtain ablack solution. Thereafter, heating was terminated, and the obtainedsolution was then cooled to room temperature. Hence, apalladium-containing composition was obtained. To the obtainedpalladium-containing composition, toluene was added to result in a totalamount of 50.0 mL, thereby obtaining a palladium-containing solution Ecomprising triphenylphosphine oxide/palladium nanocolloidal particles.The content of palladium in the palladium-containing solution E, whichwas calculated from the amount of the palladium acetate added and theamount of the solution, was 10.0 mmol/L.

Example 6

A palladium-containing solution G comprising diphenylsulfoxide/palladium nanocolloidal particles was obtained in the samemanner as that of Example 5, with the exception that 2.02 g (10 mmol) ofdiphenyl sulfoxide was used instead of 2.78 g of triphenylphosphineoxide. The content of palladium in the palladium-containing solution G,which was calculated from the amount of the palladium acetate added andthe amount of the solution, was 10.0 mmol/L.

Comparative Example 1

A palladium-containing solution I comprising oleylamine/palladiumnanocolloidal particles was obtained in the same manner as that ofExample 1, with the exception that 2.67 g (10 mmol) of oleylamine wasused instead of 2.78 g of triphenylphosphine oxide. The content ofpalladium in the palladium-containing solution I, which was calculatedfrom the amount of the palladium acetate added and the amount of thesolution, was 11.1 mmol/L.

Example 7

40 mL of the palladium-containing solution A obtained in Example 1 and90 mL of an aqueous solution, in which 0.5 mmol/L H₃PO₄ (phosphoricacid), and 2 mmol/L NaBr (sodium bromide) had been dissolved, was loadedinto an autoclave equipped with a stirrer and a gas blowing pipe, so asto obtain a mixed solution. At this time, the ratio between the waterlayer and the organic layer (the aqueous solution/thepalladium-containing solution; volume ratio) was 2.25.

Subsequently, under a nitrogen atmosphere, the autoclave was pressurizedto 10 atm. While stirring the mixed solution at 1000 rpm, a mixed gas(H₂=10% by volume, O₂=18% by volume, and N₂=72% by volume) wasdistributed at 20° C. at a flow rate of 250 cc/min.

After the reaction had been performed for 2 hours, the pressure wasreduced. Thereafter, the concentration of hydrogen peroxide in the waterlayer and the amount of hydrogen contained in discharged gas from thereaction system were measured. From these measurement results, the speedof generating hydrogen peroxide, hydrogen conversion rate, andselectivity were calculated. The results are shown in Table 1.

Examples 8 to 10 and Comparative Example 2

Hydrogen peroxide was produced in the same manner as that of Example 7,with the exception that the palladium-containing solutions B to Dobtained in Examples 2 to 4 and the palladium-containing solution Iobtained in Comparative Example 1 were each used, instead of thepalladium-containing solution A. Thereafter, various measurements andcalculations were carried out. The results are shown in Table 1.

Example 11

100 mL of the palladium-containing solution E obtained in Example 5 and170 mL of an aqueous solution comprising 10 mmol/L NaBr (sodium bromide)was loaded into an autoclave equipped with a stirrer and a gas blowingpipe, so as to obtain a mixed solution. At this time, the ratio betweenthe water layer and the organic layer (the aqueous solution/thepalladium-containing solution; volume ratio) was 1.70.

Subsequently, under a nitrogen atmosphere, the autoclave was pressurizedto 10 atm. While stirring the mixed solution at 1000 rpm, a mixed gas(H₂=10% by volume, O₂=18% by volume, and N₂=72% by volume) wasdistributed at 20° C. at a flow rate of 250 cc/min.

After the reaction had been performed for 2 hours, the pressure wasreduced. Thereafter, the concentration of hydrogen peroxide in the waterlayer and the amount of hydrogen contained in discharged gas from thereaction system were measured. From these measurement results, the speedof generating hydrogen peroxide, hydrogen conversion rate, andselectivity were calculated. The results are shown in Table 1.

Example 12

45 mL of the palladium-containing solution E obtained in Example 5 and225 mL of an aqueous solution comprising 10 mmol/L NaBr (sodium bromide)was loaded into an autoclave equipped with a stirrer and a gas blowingpipe, so as to obtain a mixed solution. At this time, the ratio betweenthe water layer and the organic layer (the aqueous solution/thepalladium-containing solution; volume ratio) was 5.00.

Subsequently, under a nitrogen atmosphere, the autoclave was pressurizedto 10 atm. While stirring the mixed solution at 1000 rpm, a mixed gas(H₂=10% by volume, O₂=18% by volume, and N₂=72% by volume) wasdistributed at 20° C. at a flow rate of 250 cc/min.

After the reaction had been performed for 2 hours, the pressure wasreduced. Thereafter, the concentration of hydrogen peroxide in the waterlayer and the amount of hydrogen contained in discharged gas from thereaction system were measured. From these measurement results, the speedof generating hydrogen peroxide, hydrogen conversion rate, andselectivity were calculated. The results are shown in Table 1.

Example 13

Hydrogen peroxide was produced in the same manner as that of Example 11,with the exception that the palladium-containing solution G obtained inExamples 6 was used instead of the palladium-containing solution E.Thereafter, various measurements and calculations were carried out. Theresults are shown in Table 1.

Example 14

To a two-necked eggplant shaped flask, 786 mg of palladium acetate (3.5mmol; molecular weight of palladium acetate: 224.51) and 2.02 g ofdiphenyl sulfoxide (10 mmol; molecular weight of diphenyl sulfoxide:202.79) were added. Further, 4.0 mL of chloroform was added thereto, andthe obtained mixture was then stirred at 50° C. for dissolution.Subsequently, 3.2 mL of oleic acid was added to the reaction mixture,and the obtained mixture was then stirred at 50° C. for 1 hour whileheating to obtain a black solution. Thereafter, heating was terminated,and the obtained solution was then cooled to room temperature. Hence, apalladium-containing composition was obtained. To the obtainedpalladium-containing composition, toluene was added to result in a totalamount of 100 mL, thereby obtaining a palladium-containing solution Jcomprising diphenyl sulfoxide/palladium nanocolloidal particles. Thecontent of palladium in the palladium-containing solution J, which wascalculated from the amount of the palladium acetate added and the amountof the solution (the aforementioned “total amount”), was 35.0 mmol/L.

Example 15

A palladium-containing solution K comprising diphenyl sulfone/palladiumnanocolloidal particles was obtained in the same manner as that ofExample 14, with the exception that 2.18 g of diphenyl sulfone (10 mmol;molecular weight of diphenyl sulfone: 218.27) was used instead of 2.02 gof diphenyl sulfoxide. The content of palladium in thepalladium-containing solution K, which was calculated from the amount ofthe palladium acetate added and the amount of the solution, was 35.0mmol/L.

Example 16

100 mL of the palladium-containing solution J obtained in Example 14 and170 mL of an aqueous solution (the pH of which was adjusted to pH 1 byaddition of sulfuric acid), in which NaBr (10 mmol/L) had beendissolved, was loaded into an autoclave equipped with a stirrer and agas blowing pipe, so as to obtain a mixed solution. At this time, theratio between the water layer and the organic layer (the aqueoussolution/the palladium-containing solution; volume ratio) was 1.70.

Subsequently, under a nitrogen atmosphere, the autoclave was pressurizedto 10 atm. While stirring the mixed solution at 1000 rpm, a mixed gas(H₂=10% by volume, O₂=18% by volume, and N₂=72% by volume) wasdistributed at 20° C. at a flow rate of 250 cc/min.

After the reaction had been performed for 2 hours, the pressure wasreduced. Thereafter, the concentration of hydrogen peroxide in the waterlayer and the amount of hydrogen contained in discharged gas from thereaction system were measured. From these measurement results, the speedof generating hydrogen peroxide, hydrogen conversion rate, andselectivity were calculated. The results are shown in Table 1.

Example 17

Hydrogen peroxide was produced in the same manner as that of Example 16,with the exception that the palladium-containing solution K obtained inExample 15 was used instead of the palladium-containing solution J.Thereafter, various measurements and calculations were carried out. Theresults are shown in Table 1.

Example 18

Hydrogen peroxide was produced in the same manner as that of Example 16,with the exceptions that the palladium-containing solution K obtained inExample 15 was used instead of the palladium-containing solution J, andthat 170 mL of an aqueous solution (the pH of which was adjusted to pH 2by addition of sulfuric acid), in which NaBr (10 mmol/L) had beendissolved, was used. Thereafter, various measurements and calculationswere carried out. The results are shown in Table 1.

Example 19

Hydrogen peroxide was produced in the same manner as that of Example 16,with the exceptions that the palladium-containing solution K obtained inExample 15 was used instead of the palladium-containing solution J, andthat 170 mL of an aqueous solution (the pH of which was adjusted to pH 3by addition of sulfuric acid), in which NaBr (10 mmol/L) had beendissolved, was used. Thereafter, various measurements and calculationswere carried out. The results are shown in Table 1.

Example 20

To a two-necked eggplant shaped flask, 786 mg (3.5 mmol) of palladiumacetate, 49 mg of bis(acetylacetonato)platinum (II) (purity: 97% bymass, 0.12 mmol, C₁₀H₁₄O₄Pt), and 2.18 g (10 mmol) of diphenyl sulfonewere added. Besides, palladium:platinum (moles)=96.8:3.2. Further, 4.0mL of chloroform was added thereto, and the obtained mixture was thenstirred at 50° C. for dissolution. Subsequently, 3.2 mL of oleic acidwas added to the reaction mixture, and the obtained mixture was thenstirred at 50° C. for 1 hour while heating to obtain a black solution.Thereafter, heating was terminated, and the obtained solution was thencooled to room temperature. Hence, a palladium-platinum containingcomposition was obtained. To the obtained palladium-platinum containingcomposition, toluene was added to result in a total amount of 100 mL,thereby obtaining a palladium-platinum containing solution L comprisingdiphenyl sulfoxide/palladium nanocolloidal particles. The content ofpalladium in the palladium-containing solution L, which was calculatedfrom the amount of the palladium acetate added and the amount of thesolution (the aforementioned “total amount”), was 35.0 mmol/L.

Example 21

Hydrogen peroxide was produced in the same manner as that of Example 16,with the exception that the palladium-platinum containing solution Lobtained in Examples 20 was used instead of the palladium-containingsolution J. Thereafter, various measurements and calculations werecarried out. The results are shown in Table 1.

Example 22

Hydrogen peroxide was produced in the same manner as that of Example 16,with the exceptions that the palladium-containing solution K obtained inExample 15 was used instead of the palladium-containing solution J, andthat 170 mL of an aqueous solution (the pH of which was not adjusted),in which NaBr (10 mmol/L) had been dissolved, was used. Thereafter,various measurements and calculations were carried out. The results areshown in Table 1.

TABLE 1 Reaction conditions for production of hydrogen peroxide Amountof Palladium-containing solution palladium- Aqueous Speed of Coatingagent containing Aqueous solution solution/ generating Hydrogen PdPlatinum Name of solution Amount Phosphoric pH palladium- hydrogenconversion concentration concentration substance Structure used usedacid NaBr adjustment pH containing peroxide rate Selectivity [mmol/L][mmol/L] [−] [−] [ml] [ml] [mmol/L] [mmol/L] [−] [−] solution [mmol/h][%] [%] Comp. Ex. 2 I 11.1 0.00 Oleylamine

 40  90 0.5  2 No — 2.25  0.15 23  1.1 Ex. 10 D 0.00 trans,trans-1,5-Diphenyl- 1,4-pentadien- 3-one

 0.41 19  3.6 Ex. 8 B 0.00 Trioctyl- phosphine oxide

 0.21 10  3.5 Ex. 9 C 0.00 Methyl (diphenyl) phosphine oxide

2.6 85  4.9 Ex. 7 A 0.00 Triphenyl- phosphine oxide

3.6 92  6.3 Ex. 13 G 10.0 0.00 Diphenyl sulfoxide

100 170 0   10 No — 1.70 6.9 56 19   Ex. 11 Ex. 12 E 0.00 0.00Triphenyl- phosphine oxide

100  45 170 225 0   0   10 10 No No — — 1.70 5.00 6.8 4.6 88 73 14  10   Ex. 16 J 35.0 0.00 Diphenyl sulfoxide

100 170 0   10 Yes 1 1.70 13.8 85 53   Ex. 17 Ex. 18 Ex. 19 Ex. 21 Ex.22 K K K L K 0.00 0.00 0.00 1.12 0.00 Diphenyl sulfone

100 170 0   10 Yes Yes Yes Yes No 1 2 3 1 — 1.70 18.2  15.0  14.0  18.7 10.9  86 86 85 87 74 63   60   55   66   18  

The invention claimed is:
 1. A palladium-containing composition comprising palladium particles and a coating agent that coats a surface of the palladium particles, wherein the coating agent comprises at least one compound selected from the group consisting of trans,trans-1,5-diphenyl-1,4-pentadien-3-one, and diphenyl sulfone.
 2. The palladium-containing composition of claim 1, wherein the at least one compound is diphenyl sulfoxide or diphenyl sulfone.
 3. The palladium-containing composition of claim 2, wherein the palladium particles are palladium colloidal particles having a particle diameter of 1 to 10 nm.
 4. The palladium-containing composition of claim 2, which further comprises platinum particles.
 5. The palladium-containing composition of claim 4, wherein the platinum particles are platinum colloidal particles, and the platinum colloidal particles are coated with the coating agent.
 6. The palladium-containing composition of claim 1, wherein the palladium particles are palladium colloidal particles.
 7. The palladium-containing composition of claim 6, wherein a particle diameter of the palladium colloidal particles is 1 to 10 nm.
 8. The palladium-containing composition of claim 1, which further comprises platinum particles.
 9. The palladium-containing composition of claim 8, wherein the platinum particles are platinum colloidal particles, and the platinum colloidal particles are coated with the coating agent.
 10. A palladium-containing solution comprising the palladium-containing composition of claim 1 and an organic solvent.
 11. The palladium-containing solution of claim 10, wherein a concentration of palladium is 5 mmol/L or more. 